1. Field of the Invention
The present invention relates to image processing for improving a viewing density of an image and for reducing the amounts of consumed color materials.
2. Description of the Related Art
As an apparatus for forming an image on a printing medium (to be also referred to as “printing paper” hereinafter), an image forming apparatus for executing image formation based on an electrophotography system (electrophotographic printer) is known. The electrophotographic printer forms an image by transferring toners as color materials onto a printing medium, and fixing the toners on the printing medium by heating and pressing the transferred toners.
In recent years, the use application of the electrophotographic printer is extended from a normal copying machine and printer to POD (Print On Demand) as a light printing range. Accordingly, a toner consumption amount reduction requirement to reduce running cost, and a further image quality enhancement requirement to improve the worth of a printed product itself are increasing.
A wide variety of types of printing media are used for the electrophotographic printer. For example, plain paper (high-quality paper, recycled paper, etc.) is used in an office, and actual printing paper (art paper, coat paper, lightweight coat paper, etc.) is used in POD. Various kinds of such paper which have paper weights as weights per unit area ranging from about 50 g/m2 to 300 g/m2 or more are available, and are set as supported paper in various electrophotographic printers.
In general, as paper has a lower paper weight, it has higher transmittance (transmissivity). Paper having high transmittance causes a phenomenon that when printed paper sheets are stacked, an image printed on an underlying paper is seen through, and when paper is viewed from a backside (reverse) face side, an image printed on a front (obverse) face is seen through (to be referred to as “show-through” hereinafter).
Various techniques have been proposed to suppress occurrence of show-through when double-sided printing is executed using printing paper having high transmittance. As one of these techniques, a corrected image is generated by multiplying, by correction coefficients, an image obtained by mirror-reversing an image to be printed on a backside face of printing paper (to be referred to as “backside image” hereinafter), and pixel values of the corrected image are subtracted from those of an image to be printed on a front face of the printing paper (to be referred to as “front image” hereinafter). Also, in another technique, after one face of printing paper is printed, transmittance of that printing paper is detected, and when the transmittance is high, one of processes for “inhibiting double-sided printing”, “changing an image density”, and “changing a fixing temperature” is executed. Furthermore, in still another technique, after one face of printing paper is printed, transmittance of the printing paper is detected, and an exposure amount upon printing an image on the other face is controlled.
As a problem caused by the transmittance of printing paper, not only the aforementioned show-through but also a problem of a viewing density change is posed. With this problem, transmitted light intensity difference from the backside face due to transmittance difference influences densities and colors viewed on a printed product.
A general viewing environment of a printed product includes light which is reflected by a wall, desk, or the like and enters the backside face in addition to directly illuminated light on a viewing face of a printed product. This phenomenon will be described below with reference to FIG. 1.
FIG. 1 is a conceptual view of a state in which a printed product is illuminated with light when viewed from a side sectional direction of the printed product. In FIG. 1, the printed product includes printing paper 101 and a toner layer 102 fixed on that paper. A light ray 103 comes from an illumination such as a ceiling illumination or illumination stand and directly enters on a viewing face (front face) of the printed product. A light ray 104 is reflected by a wall, desk, or the like, and enters the backside face of the printed product. As shown in FIG. 1, the light ray 103, which enters the front face of the printed product, is absorbed or scattered by the toner layer 102, or is transmitted through the toner layer 102 and is reflected by the front face of the printing paper 101, and is viewed as a reflected light ray 105 from the printed product.
The light ray 104, which enters the backside face of the printed product, is transmitted through the printing paper 101 and toner layer 102, and is viewed as a transmitted light ray 106. As will be described in detail later, a light ray 107 is scattered by the toner layer 102, and returns to the backside face of the printed product. Light intensity of the transmitted light ray 106 from the backside face to the front face depends on transmittance of the printing paper 101, and increases with increasing transmittance. Light intensity, which is actually viewed by the user as the printed product formed by the toner layer 102, includes that of the reflected light ray 105 and that of the transmitted light ray 106. Therefore, printing paper having higher transmittance has larger light intensity, and a density (viewing density) viewed as the toner layer 102 consequently lowers.
Also, the transmitted light intensity from the backside face to the front face of the printed product varies depending not only on the transmittance of the printing paper but also that of toner fixed on the printing paper. The transmittance of toner varies depending on a fixing state of toner although the mounted amount (applied amount) of that toner (a weight of toner per unit area) remains the same. This is because a void ratio and spatial density of pigment in the toner layer change depending on heat and pressure differences in a fixing process, and degrees of absorption and scattering of light on the toner layer change.
As described above, image quality deterioration caused by the transmittance of printing paper includes the show-through and viewing density change. As a technique for suppressing image quality deterioration, a technique for taking a measure against the show-through like in the aforementioned technique has been proposed. However, a technique for taking a measure against the viewing density change is not available.
Also, as for the toner consumption amount reduction requirement, a technique so-called a toner saving mode, which reduces a toner consumption amount at the sacrifice of a formed image density, is known. However, a technique which reduces a toner consumption amount while maintaining a viewing density of an image is not available.